Commit e77a8cf1 authored by Erik Poll's avatar Erik Poll

fixed positions of diagrams

parent 72796a3a
......@@ -9,6 +9,14 @@ SSHv2 follows a client-server paradigm. The protocol consists of three layers (F
\item The \textit{connection protocol} (RFC 4254~\cite{rfc4254}) allows the encrypted channel to be multiplexed in different channels. These channels enable a user to run multiple applications, such as terminal emulation or file transfer, over a single SSH connection.
\begin{figure}[t] % was !hb
\caption{SSH protocol layers}
Each layer has its own specific messages. The SSH protocol is interesting in that outer layers do not encapsulate inner layers. This means that different layers can interact. Hence, it makes sense to analyze SSH as a whole, instead of analyzing its constituent layers independently. Below we discuss each layer, outlining the relevant messages which are later used in learning, and characterising the so-called \textit{happy flow} that a normal protocol
run follows.
......@@ -16,19 +24,12 @@ At a high level, a typical SSH protocol run uses the three constituent protocols
%Different layers are identified by their message numbers. These message numbers will form the basis of the state fuzzing. The SSH protocol is especially interesting because outer layers do not encapsulate inner layers. This means that different layers can interact. One could argue that this is a less systematic approach, in which a programmer is more likely to make state machine-related errors.
\caption{SSH protocol components running on a TCP/IP stack.}
\subsection{Transport layer}\label{ssh-run-trans}
SSH runs over TCP, and provides end-to-end confidentiality and integrity using session keys. Once a TCP connection has been established with the server, these session keys are securely negotiated using a \textsl{key exchange} algorithm, the first step of the protocol. The key exchange begins by the two sides exchanging their preferences for the key exchange algorithm to be used, as well as encryption, compression and hashing algorithms. Preferences are sent with a \textsc{kexinit} message.
%TODO How is the algorithm picked?
Subsequently, key exchange using the negotiated algorithm takes place. Following this algorithm, one-time session keys for encryption and hashing are generated by each side, together with an identifier for the session. The main key exchange algorithm is Diffie-Hellman, which is also the only one required by the RFC. For the Diffie-Hellman scheme, \textsc{kex30} and \textsc{kex31} are exchanged to establish fresh session keys. These keys are used from the moment the \textsc{newkeys} command has been issued by both parties. A subsequent \textsc{sr\_auth} requests the authentication service. The happy flow thus consists of the succession of the three steps comprising key exchange, followed up by a successful authentication service request. The sequence is shown in Figure~\ref{fig:hf-trans}.
\caption{The happy flow for the transport layer.}
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